diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 096dd0f5..a5b02dae 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -316,6 +316,9 @@ jobs:
           fi
           echo "Generated code is up to date"
 
+      - name: Check optional-array invariant
+        run: ruby ../scripts/check-kotlin-optional-arrays.rb
+
       - name: Run Kotlin conformance tests
         run: ./gradlew :conformance:run
 
diff --git a/Makefile b/Makefile
index d5418538..9d215b88 100644
--- a/Makefile
+++ b/Makefile
@@ -743,7 +743,7 @@ tools:
 # Spec-shape lints
 #------------------------------------------------------------------------------
 
-.PHONY: check-bucket-flat-parity validate-api-gaps check-deprecation-parity
+.PHONY: check-bucket-flat-parity validate-api-gaps check-deprecation-parity kt-check-optional-arrays
 
 # Verify every bucket-scoped GET list operation has a flat-path counterpart
 # (or is justified in spec/bucket-scoped-allowlist.txt). Cross-project SDK
@@ -765,6 +765,12 @@ check-deprecation-parity: rb-build
 validate-api-gaps:
 	@./scripts/validate-api-gaps.sh
 
+# D-invariant: every optional generated Kotlin array is `List<T>? = null`, every
+# required array stays `List<T>`, and none default to the `= emptyList()`
+# sentinel — pinning the optional-array presence contract against regression.
+kt-check-optional-arrays:
+	@./scripts/check-kotlin-optional-arrays.sh
+
 #------------------------------------------------------------------------------
 # Combined targets
 #------------------------------------------------------------------------------
@@ -787,7 +793,7 @@ generate:
 	@echo "==> Generation complete"
 
 # Run all checks (Smithy + Go + TypeScript + Ruby + Kotlin + Swift + Python + Behavior Model + Conformance + Provenance + Actions lint)
-check: lint-actions sync-spec-version-check smithy-check behavior-model-check provenance-check sync-api-version-check go-check-drift go-check-wrapper-drift auth-routable-check kt-check-drift go-check ts-check rb-check kt-check swift-check py-check conformance check-bucket-flat-parity validate-api-gaps check-deprecation-parity
+check: lint-actions sync-spec-version-check smithy-check behavior-model-check provenance-check sync-api-version-check go-check-drift go-check-wrapper-drift auth-routable-check kt-check-drift go-check ts-check rb-check kt-check swift-check py-check conformance check-bucket-flat-parity validate-api-gaps check-deprecation-parity kt-check-optional-arrays
 	@echo "==> All checks passed"
 
 # Clean all build artifacts
diff --git a/kotlin/generator/src/main/kotlin/com/basecamp/sdk/generator/ModelEmitter.kt b/kotlin/generator/src/main/kotlin/com/basecamp/sdk/generator/ModelEmitter.kt
index b755d3b8..22e84b6b 100644
--- a/kotlin/generator/src/main/kotlin/com/basecamp/sdk/generator/ModelEmitter.kt
+++ b/kotlin/generator/src/main/kotlin/com/basecamp/sdk/generator/ModelEmitter.kt
@@ -191,17 +191,15 @@ class ModelEmitter(private val api: OpenApiParser) {
             "string" -> if (isRequired) "String" else "String?"
             "array" -> {
                 val itemType = resolveArrayItemType(schema["items"]?.jsonObject)
-                // Rich-text companion arrays (RichTextAttachmentList) carry a
-                // documented cross-SDK presence contract (SPEC.md §10): an
-                // OPTIONAL one must stay nullable so an absent array (a
-                // non-matching projection type or a webhook-sourced item) is
-                // distinct from a present-but-empty one, matching how Go (nil),
-                // Swift/TypeScript (optional), Python (NotRequired), and Ruby
-                // (nil) already decode it. A bare `List<X> = emptyList()` would
-                // be a sentinel for absence, which SPEC.md's Optional Fields
-                // rule forbids. Other optional arrays keep the empty-list
-                // convention (no such presence contract).
-                if (!isRequired && itemType == "RichTextAttachment") "List<$itemType>?" else "List<$itemType>"
+                // Every OPTIONAL array is nullable (`List<X>? = null`) so an
+                // absent array stays distinct from a present-but-empty one,
+                // per SPEC.md's Optional Fields rule (a sentinel — here
+                // `= emptyList()` — is not a substitute for absence). This
+                // aligns Kotlin with the other five SDKs, all of which already
+                // preserve the distinction: Go (nil pointer), Swift/TypeScript
+                // (optional), Python (NotRequired), Ruby (nil). REQUIRED arrays
+                // are always emitted, so they stay a plain non-null `List<X>`.
+                if (!isRequired) "List<$itemType>?" else "List<$itemType>"
             }
             "object" -> if (isRequired) "JsonObject" else "JsonObject?"
             else -> if (isRequired) "JsonElement" else "JsonElement?"
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CampfireLine.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CampfireLine.kt
index 64b8fc62..5c79bde1 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CampfireLine.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CampfireLine.kt
@@ -27,7 +27,7 @@ data class CampfireLine(
     val creator: Person,
     @SerialName("bookmark_url") val bookmarkUrl: String? = null,
     val content: String? = null,
-    val attachments: List<CampfireLineAttachment> = emptyList(),
+    val attachments: List<CampfireLineAttachment>? = null,
     @SerialName("boosts_count") val boostsCount: Int = 0,
     @SerialName("boosts_url") val boostsUrl: String? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Card.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Card.kt
index 1bcd8252..b1eace17 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Card.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Card.kt
@@ -38,9 +38,9 @@ data class Card(
     @SerialName("comments_url") val commentsUrl: String? = null,
     @SerialName("completion_url") val completionUrl: String? = null,
     val completer: Person? = null,
-    val assignees: List<Person> = emptyList(),
-    @SerialName("completion_subscribers") val completionSubscribers: List<Person> = emptyList(),
-    val steps: List<CardStep> = emptyList(),
+    val assignees: List<Person>? = null,
+    @SerialName("completion_subscribers") val completionSubscribers: List<Person>? = null,
+    val steps: List<CardStep>? = null,
     @SerialName("boosts_count") val boostsCount: Int = 0,
     @SerialName("boosts_url") val boostsUrl: String? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardColumn.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardColumn.kt
index b6e6c21f..38bef1b6 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardColumn.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardColumn.kt
@@ -32,6 +32,6 @@ data class CardColumn(
     @SerialName("cards_count") val cardsCount: Int = 0,
     @SerialName("comments_count") val commentsCount: Int = 0,
     @SerialName("cards_url") val cardsUrl: String? = null,
-    val subscribers: List<Person> = emptyList(),
+    val subscribers: List<Person>? = null,
     @SerialName("on_hold") val onHold: CardColumnOnHold? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardStep.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardStep.kt
index 442ff237..e24f9345 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardStep.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardStep.kt
@@ -31,6 +31,6 @@ data class CardStep(
     val completed: Boolean = false,
     @SerialName("completed_at") val completedAt: String? = null,
     val completer: Person? = null,
-    val assignees: List<Person> = emptyList(),
+    val assignees: List<Person>? = null,
     @SerialName("completion_url") val completionUrl: String? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardTable.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardTable.kt
index 2e765a26..e3f7cc62 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardTable.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/CardTable.kt
@@ -26,7 +26,7 @@ data class CardTable(
     val creator: Person,
     @SerialName("bookmark_url") val bookmarkUrl: String? = null,
     @SerialName("subscription_url") val subscriptionUrl: String? = null,
-    val subscribers: List<Person> = emptyList(),
-    val lists: List<CardColumn> = emptyList(),
-    val wormholes: List<Wormhole> = emptyList()
+    val subscribers: List<Person>? = null,
+    val lists: List<CardColumn>? = null,
+    val wormholes: List<Wormhole>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ClientApproval.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ClientApproval.kt
index 52d08452..ec3d4518 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ClientApproval.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ClientApproval.kt
@@ -35,5 +35,5 @@ data class ClientApproval(
     @SerialName("replies_url") val repliesUrl: String? = null,
     @SerialName("approval_status") val approvalStatus: String? = null,
     val approver: Person? = null,
-    val responses: List<ClientApprovalResponse> = emptyList()
+    val responses: List<ClientApprovalResponse>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/EventDetails.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/EventDetails.kt
index d9512cf9..5528ad8e 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/EventDetails.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/EventDetails.kt
@@ -12,7 +12,7 @@ import kotlinx.serialization.json.JsonObject
  */
 @Serializable
 data class EventDetails(
-    @SerialName("added_person_ids") val addedPersonIds: List<Long> = emptyList(),
-    @SerialName("removed_person_ids") val removedPersonIds: List<Long> = emptyList(),
-    @SerialName("notified_recipient_ids") val notifiedRecipientIds: List<Long> = emptyList()
+    @SerialName("added_person_ids") val addedPersonIds: List<Long>? = null,
+    @SerialName("removed_person_ids") val removedPersonIds: List<Long>? = null,
+    @SerialName("notified_recipient_ids") val notifiedRecipientIds: List<Long>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/HillChart.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/HillChart.kt
index fc57948b..554e05c9 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/HillChart.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/HillChart.kt
@@ -17,5 +17,5 @@ data class HillChart(
     @SerialName("updated_at") val updatedAt: String? = null,
     @SerialName("app_update_url") val appUpdateUrl: String? = null,
     @SerialName("app_versions_url") val appVersionsUrl: String? = null,
-    val dots: List<HillChartDot> = emptyList()
+    val dots: List<HillChartDot>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Project.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Project.kt
index e7cc5383..ed5f7b94 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Project.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Project.kt
@@ -26,7 +26,7 @@ data class Project(
     @SerialName("end_date") val endDate: String? = null,
     @SerialName("clients_enabled") val clientsEnabled: Boolean = false,
     @SerialName("bookmark_url") val bookmarkUrl: String? = null,
-    val dock: List<DockItem> = emptyList(),
+    val dock: List<DockItem>? = null,
     val bookmarked: Boolean = false,
     @SerialName("client_company") val clientCompany: ClientCompany? = null,
     @Deprecated("This shape is deprecated since 2024-01: Use Client Visibility feature instead")
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/QuestionSchedule.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/QuestionSchedule.kt
index 8b76611a..e4493f46 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/QuestionSchedule.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/QuestionSchedule.kt
@@ -13,7 +13,7 @@ import kotlinx.serialization.json.JsonObject
 @Serializable
 data class QuestionSchedule(
     val frequency: String? = null,
-    val days: List<Int> = emptyList(),
+    val days: List<Int>? = null,
     val hour: Int = 0,
     val minute: Int = 0,
     @SerialName("week_instance") val weekInstance: Int = 0,
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ScheduleEntry.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ScheduleEntry.kt
index 7453dc3b..6dc366b2 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ScheduleEntry.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/ScheduleEntry.kt
@@ -35,7 +35,7 @@ data class ScheduleEntry(
     @SerialName("all_day") val allDay: Boolean = false,
     @SerialName("starts_at") val startsAt: String? = null,
     @SerialName("ends_at") val endsAt: String? = null,
-    val participants: List<Person> = emptyList(),
+    val participants: List<Person>? = null,
     @SerialName("boosts_count") val boostsCount: Int = 0,
     @SerialName("boosts_url") val boostsUrl: String? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Subscription.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Subscription.kt
index 95131577..a6dd7b49 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Subscription.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Subscription.kt
@@ -15,5 +15,5 @@ data class Subscription(
     val subscribed: Boolean,
     val count: Int,
     val url: String,
-    val subscribers: List<Person> = emptyList()
+    val subscribers: List<Person>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Template.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Template.kt
index dcbf8c04..576ada3a 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Template.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Template.kt
@@ -20,5 +20,5 @@ data class Template(
     val description: String? = null,
     val url: String? = null,
     @SerialName("app_url") val appUrl: String? = null,
-    val dock: List<DockItem> = emptyList()
+    val dock: List<DockItem>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Todo.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Todo.kt
index 4bba0925..a0773850 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Todo.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Todo.kt
@@ -36,10 +36,10 @@ data class Todo(
     val completed: Boolean = false,
     @SerialName("starts_on") val startsOn: String? = null,
     @SerialName("due_on") val dueOn: String? = null,
-    val assignees: List<Person> = emptyList(),
-    @SerialName("completion_subscribers") val completionSubscribers: List<Person> = emptyList(),
+    val assignees: List<Person>? = null,
+    @SerialName("completion_subscribers") val completionSubscribers: List<Person>? = null,
     @SerialName("completion_url") val completionUrl: String? = null,
     @SerialName("boosts_count") val boostsCount: Int = 0,
     @SerialName("boosts_url") val boostsUrl: String? = null,
-    val steps: List<CardStep> = emptyList()
+    val steps: List<CardStep>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Webhook.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Webhook.kt
index 03f8f020..8d2fd39b 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Webhook.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated/models/Webhook.kt
@@ -19,6 +19,6 @@ data class Webhook(
     val url: String,
     @SerialName("app_url") val appUrl: String,
     val active: Boolean = false,
-    val types: List<String> = emptyList(),
-    @SerialName("recent_deliveries") val recentDeliveries: List<WebhookDelivery> = emptyList()
+    val types: List<String>? = null,
+    @SerialName("recent_deliveries") val recentDeliveries: List<WebhookDelivery>? = null
 )
diff --git a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/services/TodosService.kt b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/services/TodosService.kt
index 2baf3252..c59d495e 100644
--- a/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/services/TodosService.kt
+++ b/kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/services/TodosService.kt
@@ -94,8 +94,8 @@ class TodosService(client: AccountClient) :
     private fun fieldsFromTodo(todo: Todo): TodoFields = TodoFields(
         content = todo.content,
         description = todo.description ?: "",
-        assigneeIds = todo.assignees.map { it.id },
-        completionSubscriberIds = todo.completionSubscribers.map { it.id },
+        assigneeIds = todo.assignees.orEmpty().map { it.id },
+        completionSubscriberIds = todo.completionSubscribers.orEmpty().map { it.id },
         dueOn = todo.dueOn ?: "",
         startsOn = todo.startsOn ?: "",
     )
diff --git a/kotlin/sdk/src/commonTest/kotlin/com/basecamp/sdk/WormholesServiceTest.kt b/kotlin/sdk/src/commonTest/kotlin/com/basecamp/sdk/WormholesServiceTest.kt
index b9e589f2..1b4ac58a 100644
--- a/kotlin/sdk/src/commonTest/kotlin/com/basecamp/sdk/WormholesServiceTest.kt
+++ b/kotlin/sdk/src/commonTest/kotlin/com/basecamp/sdk/WormholesServiceTest.kt
@@ -276,11 +276,12 @@ class WormholesServiceTest {
         val account = client.forAccount("12345")
         val cardTable = account.cardTables.get(cardTableId = 1069479345)
 
-        assertEquals(2, cardTable.wormholes.size)
-        assertTrue(cardTable.wormholes[0].linked)
-        assertNotNull(cardTable.wormholes[0].destinationUrl)
-        assertTrue(!cardTable.wormholes[1].linked)
-        assertNull(cardTable.wormholes[1].destinationUrl)
+        val wormholes = assertNotNull(cardTable.wormholes)
+        assertEquals(2, wormholes.size)
+        assertTrue(wormholes[0].linked)
+        assertNotNull(wormholes[0].destinationUrl)
+        assertTrue(!wormholes[1].linked)
+        assertNull(wormholes[1].destinationUrl)
 
         client.close()
     }
diff --git a/rubric-audit.json b/rubric-audit.json
index af8c27d8..eb5ab945 100644
--- a/rubric-audit.json
+++ b/rubric-audit.json
@@ -12,8 +12,9 @@
       "note": "Request/response types generated from OpenAPI schema"
     },
     "1B.4": {
-      "pass": true,
-      "note": "Optional fields use pointers (Go), optional chaining (TS), nilable (Ruby)"
+      "pass": false,
+      "issue": 436,
+      "note": "Partially met. Optional OBJECT/REF/ARRAY and nullable-typed fields preserve absence across all six SDKs: Go pointers (*T / *[]T), TS optional chaining (?: T), Ruby nilable, Python NotRequired[T], Swift optionals (T?), Kotlin nullable (T? = null, incl. List<T>? = null for optional arrays after #433, pinned by check-kotlin-optional-arrays). NOT yet met for optional non-array SCALARS: Kotlin emits them with zero-value sentinels (Boolean = false, Int = 0, Long = 0L) — a static-type violation of SPEC.md's Optional Fields rule (0/'' are not substitutes for absence). Go optional NON-POINTER scalars collapse absence on DECODE too (an absent field and an explicit zero both unmarshal to the zero value); only Go pointer fields (*T) preserve it. Ruby/Swift decode faithfully but drop nil on RE-ENCODE (compact / synthesized encoder). All documented in SPEC.md §10 and tracked in #436. Go's *[]T is used only where rich-text projections need the wire distinction round-tripped both directions; not a blanket sweep."
     },
     "1B.5": {
       "pass": true,
diff --git a/scripts/check-kotlin-optional-arrays.rb b/scripts/check-kotlin-optional-arrays.rb
new file mode 100755
index 00000000..ececf0f6
--- /dev/null
+++ b/scripts/check-kotlin-optional-arrays.rb
@@ -0,0 +1,142 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# D-invariant guard for the Kotlin generator's optional-array contract.
+#
+# Every OPTIONAL generated array must be `List<T>? = null` so an absent array
+# stays distinct from a present-but-empty one (SPEC.md Optional Fields rule),
+# aligning Kotlin with the other five SDKs. Every REQUIRED array stays a plain
+# non-null `List<T>`. No generated array may default to the `= emptyList()`
+# sentinel that this fix removed.
+#
+# Two levels of enforcement:
+#
+#   * Schema-checked (generated/models/<Component>.kt whose basename is an
+#     OpenAPI component): each array property is cross-checked against the
+#     component's `required` set — a REQUIRED array MUST be non-null `List<T>`
+#     (no `?`, no default); an OPTIONAL array MUST be `List<T>? = null`. This
+#     proves both halves of the guarantee, so a required array mistakenly
+#     emitted as `List<T>?` is caught, not silently accepted.
+#   * Structural (everything else — request/param types in generated/services,
+#     supporting model types not present as components): a property with a
+#     default MUST be `List<T>? = null` (so `= emptyList()` or any non-null
+#     default fails). Requiredness can't be derived here, so a bare `List<T>`
+#     or nullable-without-default is left alone.
+#
+# Pins the D fix against regression. Wired into `make check`.
+
+require "json"
+
+PROJECT_ROOT = File.expand_path("..", __dir__)
+GENERATED_DIR = File.join(
+  PROJECT_ROOT,
+  "kotlin/sdk/src/commonMain/kotlin/com/basecamp/sdk/generated"
+)
+MODELS_DIR = File.join(GENERATED_DIR, "models")
+OPENAPI_FILE = File.join(PROJECT_ROOT, "openapi.json")
+
+unless File.file?(OPENAPI_FILE)
+  warn "ERROR: openapi.json not found at #{OPENAPI_FILE}"
+  exit 2
+end
+
+# Read as UTF-8 regardless of process locale (LC_ALL=C would otherwise read as
+# US-ASCII and choke on the spec's / generated code's UTF-8 bytes).
+components = JSON.parse(File.read(OPENAPI_FILE, encoding: "UTF-8")).dig("components", "schemas") || {}
+
+errors = []
+
+files = Dir.glob(File.join(GENERATED_DIR, "**", "*.kt")).sort
+if files.empty?
+  warn "ERROR: no generated Kotlin files found under #{GENERATED_DIR}"
+  exit 2
+end
+
+# Parse one property line into [wire_name, type_part, default_part] or nil when
+# the line isn't an array-typed `val`. `wire_name` is the @SerialName value when
+# present, else the Kotlin field name (the generator only adds @SerialName when
+# the camelCase name differs from the wire name, so this recovers the wire name).
+def parse_array_property(line)
+  return nil unless line =~ /:\s*(List<.*)$/
+
+  decl = Regexp.last_match(1).strip.sub(/[,)]\s*$/, "").strip
+  field = line[/\bval\s+(\w+)\s*:/, 1]
+  return nil unless field
+
+  serial = line[/@SerialName\("([^"]+)"\)/, 1]
+  wire = serial || field
+
+  type_part, default_part =
+    if decl.include?("=")
+      left, right = decl.split("=", 2)
+      [left.strip, right.strip]
+    else
+      [decl, nil]
+    end
+
+  [wire, type_part, default_part]
+end
+
+files.each do |path|
+  rel = path.sub("#{PROJECT_ROOT}/", "")
+  component_name =
+    if path.start_with?("#{MODELS_DIR}/")
+      base = File.basename(path, ".kt")
+      base if components.key?(base)
+    end
+  component = component_name ? components[component_name] : nil
+  required_fields = component ? (component["required"] || []) : nil
+  known_props = component ? (component["properties"] || {}) : nil
+
+  File.foreach(path, encoding: "UTF-8").with_index(1) do |line, lineno|
+    parsed = parse_array_property(line)
+    next unless parsed
+
+    wire, type_part, default_part = parsed
+    nullable = type_part.end_with?("?")
+    has_default = !default_part.nil?
+
+    # Schema-checked path: the field is a known property of an OpenAPI component.
+    if required_fields && known_props&.key?(wire)
+      if required_fields.include?(wire)
+        if nullable || has_default
+          errors << "#{rel}:#{lineno}: `#{wire}` is REQUIRED in schema " \
+                    "`#{component_name}` but is emitted as `#{type_part}" \
+                    "#{has_default ? " = #{default_part}" : ''}` — a required " \
+                    "array must be non-null `List<T>` with no default"
+        end
+      elsif !nullable || default_part != "null"
+        errors << "#{rel}:#{lineno}: `#{wire}` is OPTIONAL in schema " \
+                  "`#{component_name}` but is emitted as `#{type_part}" \
+                  "#{has_default ? " = #{default_part}" : ''}` — an optional " \
+                  "array must be `List<T>? = null`"
+      end
+      next
+    end
+
+    # Structural path: requiredness unknown (request/param types, supporting
+    # non-component models). Only a defaulted array is constrained.
+    next unless has_default
+
+    if default_part == "emptyList()"
+      errors << "#{rel}:#{lineno}: optional array uses the forbidden " \
+                "`= emptyList()` sentinel — must be `List<T>? = null`"
+    elsif !nullable
+      errors << "#{rel}:#{lineno}: non-null array `#{type_part}` carries a " \
+                "default (`= #{default_part}`) — an optional array must be " \
+                "nullable (`List<T>? = null`)"
+    elsif default_part != "null"
+      errors << "#{rel}:#{lineno}: nullable array defaults to " \
+                "`#{default_part}` — must default to `null`"
+    end
+  end
+end
+
+if errors.empty?
+  puts "==> Kotlin optional-array invariant clean — #{files.size} generated files scanned"
+  exit 0
+else
+  warn "Kotlin optional-array invariant failed:"
+  errors.each { |e| warn "  - #{e}" }
+  exit 1
+end
diff --git a/scripts/check-kotlin-optional-arrays.sh b/scripts/check-kotlin-optional-arrays.sh
new file mode 100755
index 00000000..d20dea60
--- /dev/null
+++ b/scripts/check-kotlin-optional-arrays.sh
@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# D-invariant guard: every optional generated Kotlin array is `List<T>? = null`,
+# every required array stays `List<T>`, and none default to `= emptyList()`.
+# See scripts/check-kotlin-optional-arrays.rb.
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+if ! command -v ruby >/dev/null 2>&1; then
+  echo "ERROR: ruby is required for check-kotlin-optional-arrays" >&2
+  exit 2
+fi
+
+exec ruby "$SCRIPT_DIR/check-kotlin-optional-arrays.rb"
diff --git a/spec/api-gaps/rich-text-attachments-coverage.md b/spec/api-gaps/rich-text-attachments-coverage.md
index 3381ae32..cebdadc4 100644
--- a/spec/api-gaps/rich-text-attachments-coverage.md
+++ b/spec/api-gaps/rich-text-attachments-coverage.md
@@ -135,8 +135,8 @@ present-empty, populated — round-trip faithfully in both directions: Go
 `*[]RichTextAttachment` (nil pointer omitted, non-nil pointer to `[]` re-encodes as
 `[]`), Swift `[RichTextAttachment]?`, TypeScript `?: …[]`, Python `NotRequired[…]`,
 Ruby nil-vs-`[]` (`parse_array`), and Kotlin `List<RichTextAttachment>? = null`
-(the model generator emits a nullable list for optional `RichTextAttachmentList`
-members rather than the empty-list default it uses for other optional arrays). The
+(the model generator emits `List<T>? = null` for *every* optional array, so an
+absent array is distinct from a present-but-empty one). The
 14 concrete `@required` members are always present, so they stay a plain
 non-optional list in each SDK.